Protoplanets are moon-sized planets, or larger embryos within protoplanetary discs. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide. According to planet formation theory, protoplanets perturb each other's orbits slightly and thus collide in giant impacts to gradually form the real planets.

In the case of the Solar System it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos had masses of about 1022 to 1023 kg and were a few thousand kilometres in diameter. Over the course of about 100 Ma they were involved in giant impacts with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought that initial collisions would have replaced the first "generation" of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided to create a third generation of fewer but even larger embryos. Eventually only a handful of embryos were left, which collided to complete the assembly of the planets proper. [cite book
title=An Introduction to the Solar System
pages=p. 56
coauthors=Iain Gilmour, Philip A. Bland, Elaine A. Moore, Mike Widdowson, Ian Wright
publisher=Cambridge University Press

Early protoplanets had more radioactive elements, the quantity of which has been reduced over time due to radioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier elements sank to the center; while lighter elements rose to the surface; such a process is known as planetary differentiation. Composition of some meteorites show that differentiation took place in some asteroids.

The giant impact theory proposes that Earth's Moon formed from a colossal impact of a hypothetical protoplanet, named Theia, with Earth early in the solar system's history.


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  • Protoplanet — [zu griech. prõtos „erster“] ist nach der verbreitetsten Theorie die Bezeichnung für den Vorläufer eines Planeten. Das Entwicklungsstadium steht in der Planetengeschichte zwischen den Planetesimalen und den fertigen Planeten. Inhaltsverzeichnis 1 …   Deutsch Wikipedia

  • Protoplanet — Protoplanet,   eine Materieverdichtung, die als Vorstufe eines Planeten angesehen werden kann. (Planetenentstehung) …   Universal-Lexikon

  • protoplanet — [prōt′ō plan΄it] n. in some cosmological theories, any of the hundreds of swirling clouds of dust and gas that form around a star and eventually evolve into individual planets protoplanetary [prōt΄ō plan′i ter΄ē] adj …   English World dictionary

  • protoplanet — noun Date: 1949 a hypothetical whirling gaseous mass within a giant cloud of gas and dust that rotates around a sun and is believed to give rise to a planet • protoplanetary adjective …   New Collegiate Dictionary

  • protoplanet — /proh toh plan it/, n. Astron. the collection of matter, in the process of condensation, from which a planet is formed. [1945 50; PROTO + PLANET] * * * ▪ astronomy       in astronomical theory, a hypothetical eddy in a whirling cloud of gas or… …   Universalium

  • protoplanet — noun astronomical objects, approximately the size of the Moon, formed from the mutual gravitational attraction of planetesimals; they are thought to collide with each other and slowly form planets …   Wiktionary

  • protoplanet — n. cloud of cosmic gas and dust that orbits a star and is in the process of condensing to form a planet (Astronomy) …   English contemporary dictionary

  • protoplanet — pro·to·planet …   English syllables

  • Protoplanet —    Planetesimal which is growing by attracting nebular material, and is in the process of becoming a planet …   The writer's dictionary of science fiction, fantasy, horror and mythology

  • protoplanet — |prōd.ə+ noun Etymology: prot + planet : a whirling gaseous eddy within a giant cloud of gas and dust rotating around a sun believed to give rise to a planet …   Useful english dictionary

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